Embryology of thyroid and parathyroid glands

1. EMBRYOLOGY OF THYROID AND PARATHYROID
CLANDS
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2. CONCEPT OUTLINE
• Embryology of thyroid
 embryogenic development
 embryogenic anomalies
• Embrology of Parathyroid
 embryogenic development
 embryogenic anomalies
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3. THYROID GLAND
• Developes from an endoderm thickening of cells that originate from
the 3rd branchial pouch
• Some cells originate form 4th branchial pouch
• In craniofacial development, cells move from the base of the tongue
to the point known as FORAMEN CAECUM, this is where the main
enlage (initial clustering of embryonic cells indicating the first trace of
an organ) of the thyroid develops
• From the base of the tongue the enlage descends as the THYROID
DIVERTICULUM,leaving the thyroglossal duct which is connected to
foremen caecum passing anteriorly to the hyoid and thyroid cartilage
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4. • It settles as a bilobed organ just inferior to the thyroid cartilage,one
lobe on each side of the trachea –anterolaterally.
• The gland lies partly on the cricoid cartilage .
• The two lobes are joined by an ISTHMUS which unites the lobes over
the trachea ,anterior to the 2nd and 3rd tracheal rings
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6. • The next stage in the development of thyroid gland is the appearance
of Thyroid follicles ,consists a layer of cells arranged around a big
central cavity.
• The cavity is filled with a colloid like material, consisting largely of the
protein Thyroglobulin
• Three cells become apparent
a) A cells or perifollicular endothelial cells: responsible for the blood
supply to the follicles
b) B cells or Follicullar cells: predominant cells,high affinity to iodine,
synthesize T3/T4
c) c) C cells/clear cells or parafollicular cells : produce calcitonin
A and B cells develop from the endoderm while C cells
develop from ectoderm.
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8. EMBRONIC DERANGEMENT/ANOMALIES OF
THYROID GLAND
A. Ectopic/Lingual Thyroid gland: Fails to descend from its embryonic
origin in the tongue resulting into thyroid gland being located high
in the neck or just inferior to hyoid bone
B. Accessory Thyroid glandular tissue: portion of thyroglossal duct may
persist to form thyroid tissue, may persist anywhere along the
course of the thyroglossal duct and may be functional but often of
insufficient size to maintain normal function if the thyroid gland
removed.
C. Pyramidal lobe of Thyroid gland: 50% of the thyroid gland have a
pyramidal lobe, varies in size,extends superiorly from the isthmus
usually to the left medial plane.
D. Complete or Incomplete isthmus
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10. PARATHYROID EMBROLOGY
• Normally patients have four parathyroid glands, two superior and two
inferior, measuring around 6 × 3 mm and weighing 35 to 40 g each. A fifth
supernumerary gland occurs in up to 10% of individuals.
• The inferior glands arise embryologically from the third brachial pouch and
migrate caudally with the thymus
• Their normal location is somewhat variable, with 60% located immediately
posterior and lateral to the thyroid lower poles and 40% in the cervical
portion of the thymus gland
• The superior glands arise from the fourth brachial pouch and migrate with
the thyroid. Seventy-five percent are posterior to the mid poles and 25%
posterior to the upper poles of the thyroid.
• The distinction between superior and inferior glands has surgical
implications because the inferior glands are anterior to and the superior
glands posterior to the recurrent laryngeal nerve.
• Resection of the superior glands poses a potential risk for nerve damage.
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11. EMBRYOLOGY ANOMALIES OF PARATHYROID
GLAND
• The term ectopic refers to glands that have descended to an unusual
location.
• They can be found cephalad at the carotid bifurcation, inferior in the
mediastinum and pericardium, anterior to the thyroid, and posterior
in the superior mediastinum in the tracheoesophageal groove and
paraesophageal region
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14. References.
• Clinically oriented anatomy (Keith Moore)
• The nuclear medicine, the requisites 4th edition
• usmle embryology review lectures
• www.slideshare.com14
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